DEAR DR. DONOHUE: Please write about the immune system. What does it mean? How is it damaged? Can one take supplements or injections for it? – B.H.

ANSWER: The immune system is the body’s Army, Navy, Air Force, Marines, Coast Guard and National Guard. It defends us from any foreign intruders – germs, in particular.

One of the largest branches of the immune system is white blood cells. Some white blood cells engage in hand-to-hand combat with germs. Others make antibodies and fire them at invaders like artillery shells.

Some cells in the immune system produce powerful chemicals that engulf and immobilize attackers.

Lymph nodes and the spleen are parts of the immune system. These organs filter body fluids, including blood, and strain from them anything that has made its way into the body.

Without a functioning immune system, people are subject to serious infections.

If the immune system is damaged, it sometimes can be reconstituted. For instance, some viral infections so weaken the immune system that the slightest infection can grow to monumental proportions and kill the infected person. If that person has a deficiency of antibodies, gamma globulin – prepackaged antibodies – can rescue the person from death. If the viral infection is eradicated, the immune system can spring back into action.

A depletion of white blood cells is another example of a weakened immune system. Antibiotics can kill invading bacteria, and the white blood cell count could return to normal.

There are instances when nothing allows for a rebound of the system. Some infants, for example, are born with seriously defective immune systems, and they often face early death.

What is the defect in your immune system? Too few white blood cells? Lack of antibody production? A missing spleen? Some of these conditions are transient.

DEAR DR. DONOHUE: I am writing to you from a distant continent.

My wife is 24. Her recent blood test said she is HBsAg positive. I would like to know something about this, as well as her chances of complete recovery. She has not felt anything unusual, and she believes that she is healthy and fit. She is three months pregnant. – B.K.

ANSWER: “HBsAg” stands for “hepatitis B surface antigen.” The surface antigen is a tiny part of the hepatitis B virus’s outer coat. It indicates infection with that virus.

Your wife needs more tests to see where she stands, as far as the infection goes. Those tests will tell if the infection is active, if her liver is damaged and if she has made antibodies to protect her from the virus.

Most adults recover from hepatitis B. Some, however, develop a chronic infection that can lead to liver cirrhosis and liver cancer. There are treatments for hepatitis B, if treatment is necessary.

You should be checked for infection, and so should the baby upon birth.

DEAR DR. DONOHUE: I am 78, and my problem is that I have had, off and on, blood in my semen. The urology doctor said it would go away. He gave me nothing. It happened again. Please help. – W.A.

ANSWER: Few things upset men more than finding blood in seminal fluid. Their minds race to thoughts of cancer. Rarely is blood in seminal fluid a sign of cancer. It usually comes from the breaking of small, surface blood vessels in the genital tract. A urologist has examined you and found nothing alarming. You have done all that needs to be done. You can sleep in peace. Even if it happens again, the chance that something serious is incubating is close to nil.

DEAR DR. DONOHUE: I am an active, healthy woman, 76, who has an inguinal hernia. Is it dangerous to live with it? If I have it repaired, I have the option of a groin incision or one below the navel. Which is better? – S.K.

ANSWER: All hernias, regardless of where they are, are bulges of organs or tissues through the wall that is supposed to keep those structures in place. In the case of an inguinal hernia, it’s a bulge through a defect in the abdominal wall in the groin region. The groin is the crease the upper leg makes with the lower abdomen.

Inguinal hernias are more common in men than in women because men are born with a natural defect in their abdominal walls in the groin. Women who get inguinal hernias have developed a defect in their abdominal walls from aging, from trauma or just from a fluke in their anatomy.

It has been a medical commandment that groin hernias should be repaired soon after they’re discovered. The reasons given are that hernias often enlarge with time, are more difficult to correct when they are large and can develop complications such as “incarceration” and “strangulation.” An incarcerated hernia is one that can’t be pushed back into the abdomen. A strangulated hernia is one whose blood supply has been cut off. It can become gangrenous unless operated on quickly. Neither complication happens all that often. Recently, a large study of middle-aged and older men has demonstrated that living with a hernia is a permissible choice. It doesn’t put hernia patients in grave danger. Many never require an operation.

Under-the-navel surgery is surgery done with a laparoscope. Small incisions are made for the scope and the instruments required to repair the hernia. This type of surgery is supposed to have a shorter convalescence. The advantage of standard hernia surgery is that it can be done without general anesthesia.

I’d choose the operation with which my surgeon is most experienced. If the surgeon prefers laparoscopic surgery, ask how many times he or she has performed it.